Computer systems generally use auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data are recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks.
A magnetoresistive (MR) sensor detects a magnetic field through the change of its resistance as a function of the strength and direction of the magnetic flux being sensed by the MR layer. Most current MR sensors are based on the giant magnetoresistive (GMR) effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of conduction electrons between magnetic layers separated by an electrically conductive non-magnetic spacer layer and the accompanying spin-dependent scattering that takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. The external magnetic field causes a variation in the relative orientation of the magnetic moments (magnetizations) of the magnetic layers, thereby affecting the spin-dependent transmission of conduction electrons and the measurable device resistance.
GMR sensors using at least two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve MR sensors. In a spin valve sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization pinned by exchange coupling with an antiferromagnetic (AFM) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium, i.e., the signal field. In spin valve sensors, the resistance varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the sensor and a corresponding change in the sensed current or voltage. The sensor is in a low resistive state if the two magnetizations are parallel and a high resistive state if the two magnetizations are antiparallel.
Conventional spin valve MR sensors take on several forms, including simple, antiparallel-pinned, and dual. A simple spin valve MR sensor 100 is shown in cross section in FIG. 1. A free ferromagnetic layer 105 is separated from a pinned ferromagnetic layer 103 by a non-magnetic, electrically conducting spacer layer 104. The magnetization of pinned layer 103, in this case into the paper, is fixed or pinned through exchange coupling with a pinning layer 102, which is typically an AFM material with a high Néel temperature such as NiO. A hard biasing layer 111 sets the magnetization of free layer 105, in this case in the plane of the paper, perpendicular to the magnetization of pinned layer 103, through magnetostatic coupling. A set of leads 112 contact pinned layer 103 to supply sense current to the device; the sense current is measured to detect the varying device resistance induced by the external magnetic field. One or more underlayers 101, such as tantalum, zirconium, nickel-iron, or alumina, are provided to control growth of the successive layers, and the device is typically terminated by a capping layer 106 to prevent corrosion.
In an antiparallel (AP)-pinned spin valve MR sensor, the pinned layer is replaced by a laminated structure that acts as an artificial ferrimagnet. FIG. 2 is a cross-sectional view of an AP-pinned spin valve MR sensor 120 with underlayers 121, a pinning layer 122, a conductive layer 126, a free layer 127, and a capping layer 128 that are identical to layers 101, 102, 104, 105, and 106, respectively, of FIG. 1. However, pinned layer 103 of simple spin valve 100 is replaced by an antiparallel-pinned structure 129 consisting of a ferromagnetic pinned layer 123, an antiferromagnetic coupling layer 124, and a ferromagnetic reference layer 125. The magnetizations of pinned layer 123 and reference layer 125 are coupled antiparallel to each other through coupling layer 124 and are perpendicular to the magnetization of free layer 127. AP-pinned spin valves are advantageous because the net magnetic moment of AP-pinned structure 129, the difference between the two antiparallel moments of the component ferromagnetic layers, can be varied independently of the thickness of pinned layer 123. Thus it is possible to balance the overall moment of the spin-valve while choosing a preferred thickness for the pinned layer 123. Furthermore, AP-pinned spin valves also exhibit a greatly enhanced stability in comparison with simple spin valves, since the coupling between pinned layer 123 and reference layer 125 is quite high.
A dual spin valve is a simple or AP-pinned spin valve with a second set of conductive, pinned, and antiferromagnetic layers deposited on top of it. Each of the pinned layers can be either a single ferromagnetic layer or an artificial ferrimagnetic layer as described above. Dual spin valves exhibit an enhanced sensitivity, but are much thicker and typically show a lower resistance than the previously described simple and AP-pinned spin valves.
As a real recording densities in magnetic media continue to increase, smaller magnetoresistive sensors with higher signals are required. MR signals are measured as ΔR/R, the percent change in device resistance as the ferromagnetic layer magnetizations switch between parallel and antiparallel. Specifically, as densities approach 100 Gbit/in2, the gap between the shields of the read head, in which the sensor is positioned, must decrease from current thickness of 0.1 μm to between 50 and 70 nm. Smaller sensors require thinner layers, which tend to produce lower signals. NiO pinning layers are unsatisfactory in these thickness regimes because of their low magnetic anisotropy energy, which leads to a weak pinning field and a high critical layer thickness. The low ordering temperature of NiO also causes thermally unstable pinning. As a solution to this problem, cobalt-ferrite pinning layers were introduced in co-pending U.S. patent application Ser. No. 09/755,556, filed Jan. 4, 2001, (issued as U.S. Pat. No. 6,721,144) is herein incorporated by reference.
While cobalt-ferrite provides a number of advantages over NiO and other standard AFM pinning layer materials, it also introduces two problems. First, coercive ferrites are thermally unstable in the thickness regime of approximately 30 nm or less, which is required for 50-nm gap sensors. Second, unlike AFM pinning layers, ferrites exhibit a substantial magnetic moment that contributes to the overall device moment, making it difficult to balance the device moment as required for stable and consistent operation. Thicker layers contribute a greater moment, and so reducing the pinning layer thickness while maintaining thermal stability would address both problems.
Spin valves containing an oxidized iron layer inserted at the pinned layer/NiO pinning layer interface are disclosed in R. F. C. Farrow et al., “Enhanced blocking temperature in NiO spin valves: Role of cubic spinel ferrite layer between pinned layer and NiO,” Applied Physics Letters, 77(8), 1191–1193 (2000). The iron oxide layer is converted to a cubic spinel nickel-ferrite (Ni0.8Fe2.2O4) by solid-state reaction with the NiO layer during annealing. The exchange bias field originates from both the NiO pinning layer and the nickel-ferrite layer. Nickel-ferrite has a relatively low coercivity; for example, it is generally not possible to grow nickel-ferrite with coercivities of 1 kOe. While nickel-ferrite/NiO spin valves display increased blocking temperature (temperature at which the exchange field drops to zero) and improved thermal stability, they cannot fit within a 50 nm sensor gap.
There is still a need, therefore, for an improved exchange-coupled magnetic structure that uses coercive ferrite pinning layers and remains thermally stable when reduced to the thicknesses required for magnetic read heads.